This invention relates to nonlinear optical devices and more particularly to devices for doubling the frequency of electromagnetic radiation passing therethrough.
Optical digital data storage devices, such as compact discs, have recently come into common usage. Typically, such discs are read and written to by means of a light emitted by a semiconductor laser (i.e. a laser diode). However, the light generated by semiconductor laser diodes generally falls within the lower end of the electromagnetic frequency spectrum (i.e. red or infrared). The use of higher frequency light, i.e. at the blue end of the spectrum, to read and write to optical storage medium would result in greatly increased storage density. Unfortunately, however, there are yet no practical blue semiconductor lasers. To date, the only blue lasers are large gas lasers which are obviously unsuitable for use in compact and inexpensive optical storage read/write devices.
Accordingly, a device capable of converting the light emitted by readily available semiconductor laser diodes to blue light is greatly desired. Laser diodes that emit infrared light are inexpensive and widely available. The frequency of blue light is twice that of infrared radiation. Accordingly, a device capable of doubling the frequency of infrared radiation has considerable commercial potential. The present invention is directed to providing an inexpensive frequency doubling device that may be used in conjunction with an infrared semiconductor laser to provide blue light suitable for use in reading and writing optical storage media.
The field of the non-linear optics has provided a number of devices used as frequency doublers, generally through the means of second harmonic generation (SHG) of a fundamental frequency. Such devices include bulk materials and stacks of non-linear crystals. A particularly effective doubling device is a non-linear optical waveguide. As a light beam passes through the waveguide the non-linear optical effect causes the generation of a lightwave of the second harmonic of the input lightwave. Such optical waveguides can be quite efficient in providing frequency doubling.
However, efficient frequency doubling requires accurate phase matching between the fundamental and harmonic waves. If the frequency doubling device is not properly phase matched interference effects will cause attenuation of the second harmonic. In a waveguide the tolerance requirements for accurate phase matching between the geometrical and physical properties of the waveguide are very difficult to achieve. A number of different structures have been proposed to provide phase matching. Phase matching has been attempted by both passive and active means. Passive phase matching has been accomplished by, for example, the addition of a periodic structure to a frequency doubling device. However, such devices are incapable of responding to changeable conditions and may lose accuracy over time. In contrast, the parameters of an active structure can be controlled in response to the measured output of the doubled light. Accordingly, it is desirable to be able to control the phase matching properties in an active manner. However, active phase matching devices have either been impractical or incapable of controlling the phase matching to a sufficient degree.
Proposals for active phase matching of the waveguide have been made in, for example, U.S. Pat. No. 4,427,260 (Puech et al) issued Jan. 24, 1984 and in the article "Electric Field Tuning of Second-Harmonic Generation in A Three-Dimensional LiNbO.sub.3 Optical Waveguide", Applied Physics Letters, 34(1), Jan. 1, 1979. These proposals achieve phase matching by means of electro-optic tuning of the material forming the waveguide. However, these approaches are constrained by the fact that the index of refraction of the waveguide will undergo only relatively small changes by means of the electro-optic effect. Accordingly, such means are able to compensate for only relatively small changes in the geometrical or physical properties of the waveguide.
The present application is directed to overcoming the difficulties of the prior art. Specifically, the structure and methodology of the present invention is capable of achieving phase matching over a relatively wider range of geometric and physical changes to the waveguide.